With the popularization of information communication services, the emergence of various multimedia services, and the provision of high-quality services, demand for a wireless communication service has increased. To actively cope with such demand, first of all the capacity of a communication system should be increased. To increase communication capacity in wireless communication environments, a method for newly searching available frequency bands and a method for increasing efficiency for limited resources may be considered. As to the latter method, a multiple transmit/receive antenna technique has recently drawn attention and has been actively developed. The multiple transmit/receive antenna technique obtains a diversity gain by equipping a transmitter and a receiver with a plurality of antennas to additionally ensure a spatial region for utilizing resources, or increases transmission capacity by transmitting data in parallel through the plurality of antennas.
A multiple antenna system using an orthogonal frequency division multiplexing (OFDM) requires various techniques to improve the reliability of data transmission. A scheme for raising a spatial diversity gain includes space-time coding (STC), cyclic delay diversity (CDD), etc. A scheme for increasing a signal-to-noise ratio (SNR) includes beamforming, precoding, etc. The beamforming and precoding schemes are used to maximize the SNR through corresponding feedback information in a closed-loop system which is capable of using feedback information in a transmitting end.
A MIMO precoding scheme is one MIMO system scheme for improving communication performance by feeding back a part of or all channel information using a closed-loop mode. The precoding scheme has an advantageous effect, especially in a slow fading environment of less than 30 Km per hour.
Examples of the precoding scheme in a closed-loop system are a method for quantizing channel information and feeding back the quantized information, and a codebook-based precoding method which is used when feedback information is finite. The codebook-based precoding method refers to a scheme for obtaining an SNR gain by feeding back an index of a precoding matrix which is previously known to transmitting and receiving ends to the transmitting end.
The beamforming scheme includes a general beamforming scheme, a codebook-based beamforming scheme, and a scheme using a combination of beamforming and MIMO schemes.
FIGS. 1A, 1B, and 1C are views explaining an example of a beamforming scheme and illustrate beam shapes formed through antennas when using the beamforming scheme.
FIG. 1A illustrates a beam shape formed through antennas when a general beamforming scheme is used. The general beamforming scheme refers to adaptive beamforming in which a weight vector for beamforming of a transmit antenna is updated according to direction of arrival (DoA). For example, an optimum antenna weight is calculated using a Wiener solution which can be obtained by a Wiener equation to eliminate signals received from unnecessary directions, thereby increasing the reception performance of an SNR.
FIG. 1B illustrates a beam shape formed through antennas when a codebook-based beamforming scheme is used. The codebook-based beamforming scheme has a simple configuration as shown in FIG. 1B and may correspond to fixed beamforming. The codebook-based beamforming improves performance by causing multiple users to simultaneously select an optimal beam. An example of the codebook may be a codebook based on a discrete Fourier transform (DFT) matrix.
FIG. 1C illustrates a beam shape formed through antennas when a scheme using a combination of beamforming and MIMO schemes is used. In this case, the conventional MIMO scheme and the beamforming scheme can be simultaneously supported using X-pol antennas of an X shape as illustrated in FIG. 1C.
A frequency division duplex (FDD) mode using separate frequency bands for uplink/downlink transmission has a difference in characteristics between uplink and downlink channels for calculating an antenna weight. Accordingly, when using the beamforming scheme, since an antenna weight for beamforming based on an accurate channel estimation result is not generated, a gain loss may occur. Furthermore, the above-described codebook-based beamforming scheme, and the scheme using a combination of the beamforming and MIMO schemes may generate a dead zone where a signal disappears since a user equipment (UE) becomes more distant from a corresponding beam as the user equipment moves, as can be confirmed in FIGS. 1B and 1C.
In addition, when feeding back an antenna weight to a base station from a user equipment using a closed-loop mode, excessive overhead may occur. Feedback overhead can, to some degree, be reduced using a codebook, but a quantization error, which may be generated in the course of quantization of channel information, and an error due to a feedback delay can not be disregarded. To relieve limitations of the utilization of the codebook, research into methods for reducing the quantization error, the feedback delay error, and feedback overhead by flexibly varying elements of the codebook according to variations of a channel in a time or frequency region has recently been conducted.